Adding an example on how to implement custom policy for content store

A very simplistic example node-access-policy.h illustrates how to get
pointer to the ContentStore/Node and to enumerate neighbors of the node upon each
addition and eviction of the ContentStore entry.

To run scenario, which uses new policy:
   ./waf --run=ndn-simple-with-cs-node-access

content-store-with-node-access.cc instantiates the following variants of
the ContentStore (can be used in ndn::StackHelper::SetContentStore):

- ns3::ndn::cs::LruNodeAccess
- ns3::ndn::cs::RandomNodeAccess
- ns3::ndn::cs::FifoNodeAccess
- ns3::ndn::cs::LfuNodeAccess